Variable directional coupler



July 13, 1965 R. G. VELTROP 3,195,075

VARIABLE DIRECTIONAL COUPLER Filed Aug. 20, 1962 2 Sheets-Sheet 1INVENTOR.

ROBERT G. VELTROP TTORNEY R. G. VELTROP 3,195,075

VARIABLE DIRECTIONAL COUPLER July 13, 1965 Filed Aug. 20, 1962 2Sheets-Sheet 2 Fl 5 4- INVE ROBERT G. ROP

ATTORNEY United States Patent Office 3,195,075 Patented July 13, 19653,195,075 VARIABLE DIRECTIONAL COUPLER Robert G. Veltrop, Sunnyvale,Califi, assignor to Sylvania Electric Products Inc., a corporation ofDelaware Filed Aug. 20, 1962, Ser. No. 217,978 1 Claim. (CL 333-40) Thisinvention relates to improvements in variable directional couplers. Ageneral object of this invention is the provision of an electromagneticdirectional coupler in which the spatial relationship between twoconductors in two orthogonal planes is selectively variable to permitadjustment of the center frequency of the coupling charactertisic asWell as the coupling factor.

The invention will be more fully understood from the following detaileddescription of the operation, reference being made to the accompanyingdrawings in which:

FIGURE 1 is a plan view of a coupler embodying the invention;

FIGURE 2 is a transverse section taken on lines 22 of FIGURE 1;

FIGURE 3 is an enlarged fragmentary sectional view of a modified form ofthe invention in which the ground planes are relocated in order toincrease the bandwidth of the coupler; and

FIGURE 4 is a section similar to FIGURE 3 with the position of the partsshifted to illustrate adjustment of the coupler.

The electromagnetic directional coupler illustrated in FIGURES 1 and 2comprises a stationary base plate 1 and connected back plate 2 togetherwith a movable frame F consisting of side plates 3 and 3, front plate 4,support plate 5, and upper plate 6. Plates 1, 3, 3', 4, 5 and 6 areelectrical ground planes and constitute an outer conducting enclosure.The opposite ends of a primary conductor 12 extend through upper plate 6and are connected, respectively, to an input line through terminal 10and tea direct coupled line through terminal 11. The opposite ends of asecondary conductor 9 extend through base plate 1 and are connected,respectively, to a so-called isolated line through terminal 17 and tothe coupled line through terminal 18. roximate portions of conductors 9and 12 are straight and are parallel to each other.

Longitudinal positioning of the movable frame F, FIG- URE l, adjusts theamount of longitudinal overlap of conductors 9 and 12 which overlapconstitutes the coupling section having an electrical length 6, and isachieved by rotation of longitudinal drive shaft 14. This drive shaft 14is threaded through side plates 3 and 3' and support plate 5 and isjournalled in supports 24 fixed to base plate 1. Vertical movement ofupper plate 6, as viewed in FIGURE 2, adjusts the spacing 1 betweenprimary and secondary conductors 12 and 9, and is accomplished bythreaded shaft journalled in plate 1 and engaging threads in plate 6.Manual adjustment of shafts 14 and 15 is provided by knobs 13 and 16,respectively.

The following explanation of the principle of the operation of adirectional coupler will be helpful to an understanding of theinvention. Upon connection of an external source of radio frequencyenergy to input terminal 10, a primary current i flows in primaryconductor 12. This causes a magnetic field to be set up around theconductor, the direction of the fiux lines being clockwise as viewedalong the conductor in the direction of current flow. Depending on theproximity of the second conductor to the first conductor, a number ofthese flux lines link secondary conductor 9 and induce a secondaryvoltage v,, on the conductor. This is expressed by the relationship dlv,,, M dt (1) where M is the mutual inductive coupling between theconductors, and di /alt is the rate of change of primary current withtime. The negative sign in Equation 1 indicates that the voltage inducedon the secondary line opposes the change in the primary current. Thus,when a current i flows in conductor 12 from input terminal 19 to outputterminal 11, a voltage v is induced in conductor 9 which causes acurrent 1' to flow in conductor 9 from terminal 18 to terminal 17 whenthe primary current is increasing in value and from terminal 17 toterminal 18 when the primary current is decreasing in value.

Similarly, currents i flow in secondary conductor 9 due to the voltage vimpressed on conductor 12 by the primary current i and the capacitivecoupling between the conductors. The currents i flow toward the commoncoupling section (into conductor 9 from terminal 17 and 18) when theprimary potential is increasing in value and away from the commoncoupling section (into terminal 17 and 18 from conductor 9) when theprimary potential is decreasing in value.

To summarize, the secondary currents i and i, are degrees out of phasein terminal 17 and are in phase in terminal 18 whether the primarycurrent i is increasing or decreasing. Therefore, if the coupler isdesigned such that the inductive and capacitive coupling are equal, thecurrents flowing in terminal 17 cancel and no output is obtained on theisolated line whereas the currents in terminal 18 add to provide maximumoutput on the coupled line and yield maximum directivity.

The value of the coupling characteristic, known as the coupling factor,is controlled by changing the value of mutual capacitive and inductivecoupling between conductors 9 and 12. This is accomplished withoutchanging the center frequency of the coupler by varying the spacing 1between conductors 9 and 12 by means of drive shaft 15 with its knob 16.

The voltages appearing respectively on the coupled line (terminal 18)and on the direct line (terminal 11) are:

1/1-70 cos 6+j sin 9 where k is the voltage coupling coefiicient, E isthe coupled voltage, E is the direct or output voltage, E is the inputvoltage, and 6' is the electrical length of the common coupling sectionbetween conductors 9 and 12. The coupled signal is maximum for 6:(2n-1)1r/ 2 radians, n=1, 2, 3, or when sin 0:1. The couplingcoeificient, determined by the physical relationship between theconductors and the ground plane, determines the bandwidth of thecoupler.

With a known common coupling section of electrical length 6 which is M4long at some frequency it is possible to determine the center frequencyof the coupling characteristic and the directional coupler from therelationship where f is the center frequency, A is the wavelength, and

c is the velocity of light. Adjustment of the electrical length 0 of thecommon coupling therefore makes it possible to vary the center frequencyof the coup-ling characteristic.

In the embodiment of FIGURES l and 2, the electrical length 0. of thecommon coupling section is varied by rotating knob 13 and shaft 14 toadjust the relative longitudinal displacement of conductors 9 and 12.For coupling values of less than db, the spacing of the conductors fromeach other and from ground planes is suflicient that vertical movementof the upper plate 6 does not affect the characteristic impedance of theconductors and the coupling characteristic remains substantiallyconstant as the center frequency is varied.

In the modified form of the invention shown in FIG- URES 3 and 4, theground planes of the coupler are relocated to provide broadbandoperation and to provide a constant coupling characteristic for tightcouplings. Input and output lines 22 and 23 are connected to primaryconductor 21. Ground plane 24 is located between lines 22 and 23. Theline designated by reference character 28 is the edge of ground plane 24adjacent to primary conductor 21. Ground planes 25 and 26 are locatedadjacent to lines 22 and 23, respectively. Lines 22 and 23' areconnected to secondary conductor 21'. Ground plane 24' is locatedbetween lines 22' and 23. Ground planes 25' and 26' are located adjacentto lines 22' and 23', respectively. The junctions of ground planes 25,25' and 26, 26' are designated by reference characters 27' and 27,respectively. The phantom lines 21a, 27a and 28a of FIGURE 4 representanother position to which conductor 2'1, ground plane junction 27 andedge 28 of ground plane 24, respectively, may be moved in order tochange the coupling factor; i.e., to change the tightness of thecoupling.

The relative positions of the input and output lines, the conductors andthe ground planes determine the characteristic impedance and match ofthe coupler, and these parts are adjusted relative to the conductors andwith respect to each other to obtain the proper coupling value. When thecoupled conductors 21 and 21' are longitudinally offset (in a right-leftdirection as viewed in PI"- URE 4) to change the center frequency, thecoupling factor and the characteristic impedance and match of the linesremain constant because the effective spacing (in the verticaldirection, as viewed in FIGURE 4) of lines, conductors and ground planesare constant. The spacing d between conductor 21 and ground plane 25 isinitially adjusted to provide the desired characteristic impedance andis effective for this purpose when the conductors 21 and 21 arelongitudinally offset as shown in FIGURE 4. The same is true of thespacing between conductor 21 and ground plane 26'. f

When the conductors and ground planes are moved to the positions shownby the phantom .lines in FIGURE 4, junctions 27 and 2'7 are offset by adistance X. The spacing d between conductor 21 (coincident with phantomline 21a) and ground plane junction 27a( coincident with phantom lines27a) is maintained constant at its initial value in order to maintainproper match and characteristhe center frequency of the couplingcharacteristic. Thus, the physical relationship between the ground planeand the conductors is maintained constant and adjustment of the couplingcharacteristic and the center frequency of the coupling characteristicare independent of each other.

As many modifications of this invention can be made without departingfrom its true spirit, the scope of this invention is to be determinedfrom the appended claim and not from the specific embodiment describedherein.

What is claimed is:

A variable directional coupler for coupling electromagnetic energy atmicrowavevfrequencies over a frequency band, said coupler comprising anelectrically conducting base member,

a first conductor supported on and insulated from said base member, saidfirst conductor having a straight portion 0 predetermined length spacedfrom and extending parallel to one side of said base member, anelectrically conducting frame supported on said base member for parallelmovement relative thereto and forming an enclosure therewith,

said frame comprising side plates projecting from said one side of thebase member and an upper plate having an inner side spaced from saidbase plate, I a second conductor supported on andinsulated from saidupper plate,

said second conductor having a straight portion of predetermined lengthspaced from and extending parallel to said one side of the upper plateand being parallel to and overlying said straight portion of said firstconductor in a common plane whereby the straight portions of said firstand second conductors are totally enclosed by said enclosure, means formoving said frame parallel to said base memher in a direction parallelto said straight portions of said conductors for changing the lengththat said straight portions overlie each other while maintaining saidstraight portions parallel and in the common plane whereby to shift themidband frequency of the handover which the energy is coupled, and meansfor moving said second conductor relative to said base plate and saidfirst conductor in a' direction normal to the said parallel movement toequally change the spacing between the said first and said secondconductors at all points along the said conductors whereby the amount ofenergy coupled between the conductors is changed.

3/24 Birch-Field 336l2l 1/65 Bock et al. 333-l0 JOHN F. BURNS, PrimaryExaminer.

LARAMIE E. ASKIN, Examiner.

